Interesting, did not know about that attribute. Tested some tap devices on my machine and get a maxmtu value close to max uint16, way bigger than RTE_ETHER_MAX_JUMBO_FRAME_LEN.
I do not know if will have time to code and test using netlink to set this limit, for the time being i think RTE_ETHER_MAX_JUMBO_FRAME_LEN is better than the previous limit. On 2022-08-08 18:42, Stephen Hemminger wrote: > On Mon, 8 Aug 2022 17:38:21 +0200 > Francesco Mancino <francesco.manc...@tutus.se> wrote: > >> Thank you for the feedback. >> >> I am sorry for the email spam, it was a learning process. >> >> I am not sure what results we would get by querying the MTU >> before configuring it, since we can (and do) set the MTU using >> SIOCSIFMTU. >> >> On 2022-08-08 17:03, Stephen Hemminger wrote: >>> On Mon, 8 Aug 2022 16:49:44 +0200 >>> Francesco Mancino <francesco.manc...@tutus.se> wrote: >>> >>>> eth_dev_validate_mtu, introduced in 990912e676e, validates configured >>>> MTU plus overhead against max_rx_pktlen. >>>> Since TAP is a virtual device, it should support as big MTU as possible. >>>> >>>> Signed-off-by: Francesco Mancino <francesco.manc...@tutus.se> >>> dev_info->min_rx_bufsize = 0; >>> >>> >>> Thanks for your patience. >>> >>> Since tap is built on top of an existing kernel network device a more >>> complete solution would be to query the kernel device to find out what >>> its MTU is. >>> >>> Acked-by: Stephen Hemminger <step...@networkplumber.org> >>> >> > > There is an attribute IFLA_MAX_MTU reported by devices over netlink. > > $ ip -d li show dev enp2s0 > 2: enp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq state UP mode > DEFAULT group default qlen 1000 > link/ether d8:5e:d3:06:5d:13 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu > 68 maxmtu 9194 addrgenmode none numtxqueues 1 numrxqueues 1 gso_max_size > 64000 gso_max_segs 64 gro_max_size 65536 parentbus pci parentdev 0000:02:00.0 > > Notice maxmtu of 9194 reported. > > Tap device already has a netlink infrastructure >
